Peel the onions, then roughly chop along with the carrots and celery.
Add to a large pan over a medium heat with a splash of olive oil and the herbs.
Fry for around 15 minutes, or until turning golden.
Add the sugar and continue to cook for a further 5 minutes, or until sticky and caramelised.
Stir in the flour, then add the Marmite, tomato purée and vinegar.
Pour in the stock, then bring to the boil. Reduce to a simmer for around 10 minutes, or until thickened and reduced.
Pass it through a sieve into another large pan, using a spoon to squeeze out all the flavours.